A railway switch machine is used to divert a train from one track to another track. In many cases, the switch machine is remotely operated and, thus, an operator cannot see the machine. Consequently, the status of the machine (e.g., points detected and mechanically locked for either a straight-through or turn-out move) is provided by electrical circuits that, in turn, are interlocked with signals governing movement of the trains. According to typical convention, the term Normal (N) is employed for a straight-through move and the term Reverse (R) is employed for a turn-out move.
Historically, indication circuits for switch machines were implemented with cam operated or other types of mechanical switches within the machine. In some cases, the indication contacts of one machine are electrically connected in series with other machines. All interconnected machines must prove that their points are closed and mechanically locked before railroad signals are cleared, in order to permit movement of associated trains.
Motor control is also provided by mechanical switches. Basically, the motor rotates in opposite directions for Normal and Reverse. Rotary motion of the motor is converted to linear motion within the machine to move and lock the points. If the motor is being driven Normal, then contacts within the machine open the circuit path that would, otherwise, permit continued movement in that direction when the limit of intended motion is reached. However, a path is maintained that permits movement in the Reverse direction. In between the extreme positions, both current paths are closed for movement of the motor in either direction.
It is known to assign Right Hand Points Closed (RHPC) or Left Hand Points Closed (LHPC) to Normal by orientation of cam operated switches.
With mechanical controllers, a battery voltage is fed from the wayside case to contacts of a first switch machine. Then, if those contacts are closed, the battery voltage is fed on to the next machine, and so on. If all the contacts in the, series string are closed, then the voltage fed back to the wayside case proves all switch machines are in correspondence, which is a condition necessary to clear signals.
Switch machines such as, for example, without limitation, M-3, M-23A and M23-B switch machines, are designed to detect drifting of switch points beyond a predetermined distance and in response thereto signal a point failure, thus stopping any traffic traveling along the line.
As described in Federal Railroad Administration (FRA) section 236.334 a switch machine must not give correspondence to clear a signal if the point closure is one-fourth of an inch when a latch-out device is not present or three eighths of an inch when it is present. When such separation distances are reached or exceeded, a linkage in the switch machine opens a set of contacts causing an open circuit and a loss of transmitted signal. In such instance, all train traffic along the line is stopped until a maintenance crew can physically inspect the switch machine and address/correct the cause for the undesired contact separation.
Accordingly, there remains a substantial need for a system which minimizes or eliminates such undesirable stoppages.